To enhance or increase the production of oil and gas hydrocarbons from wells bored into subterranean-formations, it has been common practice to pump a viscous fluid at high pressures down into the wellbore to crack the formation and force the fracturing fluid into those cracks. The fracturing fluid is also used to carry sand or other types of particles, called proppants, to hold the cracks open when the pressure is relieved. The cracks held open by the proppant provide additional paths for the oil or natural gas to reach the wellbore, which, in turn, increases the production of oil and/or natural gas from the well.
In order to form the viscous fluid, a thickening agent (or a viscosifying agent), such as a polymer, is incorporated into water or an aqueous solution. A number of polymers are known for this purpose including a number of polysaccharides. Viscosity can then be increased considerably, giving a viscoelastic gel, by cross-linking the polymer molecules. This has particular application in connection with the extraction of hydrocarbons such as oil and natural gas from a reservoir which is a subterranean geologic formation by means of a drilled well that penetrates the hydrocarbon-bearing reservoir formation. In this field, one commercially very significant application of thickened fluids is for hydraulic fracturing of a subterranean formation. The polymeric thickening agent may (1) assist in controlling leak-off of the fluid into the formation, (2) aid in the transfer of hydraulic fracturing pressure to the rock surfaces and (3) facilitate the suspension and transfer into the formation of proppant materials that remain in the fracture and thereby hold the fracture open when the hydraulic pressure is released.
Further applications of thickened fluids in connection with hydrocarbon extraction may include acidizing, control of fluid loss, diversion, zonal isolation, and the placing of gravel packs. Gravel packing is a process of placing a volume of particulate material, frequently coarse sand, within the wellbore and possibly extending slightly into the surrounding formation. The particulate material used to form a gravel pack may be transported into place in suspension in a thickened fluid. When it is in place, the gravel pack acts as a filter for fine particles so that they are not entrained in the produced fluid.
Crosslinking of the polymeric materials then serves to increase the viscosity and proppant carrying ability of the fluid, as well as to increase its high temperature stability. Typical crosslinking agents comprise soluble boron, zirconium, and titanium compounds. Chromium and aluminum compounds have also been used. The viscosity of solutions of guar gum and similar thickeners can be greatly enhanced by crosslinking them with boric acid or other boron containing materials. Thus, boron crosslinked guar gum solutions are useful as fracturing fluids.
Historically, as described in U.S. Pat. Nos. 6,310,104 and 6,372,805, the disclosures of which are incorporated by reference herein in their entirety, amorphous borosilicate particles in the size domain of 10-20 nm and in the concentration range of 20-40 wt % in water solvent have been used in the paper industry. The mono-dispersion is achieved by adding aqueous silicic acid to an aqueous boric oxide solution with extended agitation, followed by recovering the aqueous colloids containing amorphous, not glassy, borosilicate nano-spheres. These products have been used in paper industry to increase the conversion of trees to paper by insuring that raw material fibers used in the process are retained and become part of the final paper sheet. They also facilitate the capture of raw material fibers in the produced paper sheet and minimize the loss of value resources to the generation of waste. In addition, they enhance the removal of water from municipal sludges which reduces fuel consumption during transportation of the sludges. However, neither of the above references described that amorphous borosilicate may be used a crosslinker for a wellbore composition used to treat a subterranean formation.
The viscosity of these crosslinked gels can be reduced by mechanical shearing (i.e., they are shear thinning) but gels cross-linked with boron compounds may reform spontaneously after exposure to high shear. This property of being reversible makes boron-crosslinked gels particularly attractive and they have been widely used. Furthermore, the overall performance of a fracturing fluid intimately depends on the cross-linking chemistry that forms the viscous gel. Borate crosslinked gel fracturing fluid typically utilize the borate anion to crosslink the hydrated polysaccharide polymers and thus provide increased viscosity. The crosslinked polymer may then be rendered chemically reversible by altering the pH of the fluid system. It is this reversible characteristic of crosslinked borate polymer fluids that may improve the effectiveness of the subsequent clean up step more effectively, and thus potentially result in good regained permeability and conductivity.
It is generally desirable to achieve the desired viscosity with a low concentration of thickening materials so as to reduce cost of materials and reduce the amount of material which is delivered below ground and may need to be removed in a subsequent cleanup operation. Also, boron and metals, in sufficient concentration, can be toxic to the environment and so it is also desirable to minimize the amount of boron or metallic cross-linking agent which is used.
Additionally, it is desirable to develop a new cross-linker material that is completely free of boron or, alternatively, to use an insoluble form of boron with an identical electronic configuration of borax so that the well established boron crosslink chemistry can remain intact.